When manufacturing gears, in particular for large gear arrangements, such as gear arrangements which are suitable for use in drive trains for wind turbines, it is often necessary to manufacture, at least the teeth of the gear wheels, from a wear resistant material in order to prevent damage to the gear wheels, misalignment and uneven loads on the gear teeth during use. Normally, the entire gear wheel is manufactured from the wear resistant material. This is a disadvantage because the wear resistant material is very often expensive and possibly scarce. Accordingly, it is desirable to reduce the amount of wear resistant material used for the gear wheels. Even those expensive and possibly scarce materials as typically used in the industry will typically require an additional thermal treatment or hardening to achieve the desired strength, durability and wear resistance. Having to heat the entire gear wheel, especially for large gears as used in wind turbines, can be a significant contribution to cost, and consumes considerable amounts of energy.
To this end composite gear wheels, i.e. gear wheels which are made from two or more materials, have previously been suggested, the composite gear wheels comprising an inner part made from one material and an outer part made from a different material. The material of the inner part may be a low cost material, and the material of the outer part, which normally has the gear teeth formed therein, may be a more expensive and/or scarce material, fulfilling specific requirements, e.g. regarding durability, hardness, wear resistance, etc.
Examples of composite gear wheels are, e.g., shown in JP 58-128562, GB 462,999, GB 2 039 662, GB 2 123 317 and U.S. Pat. No. 4,070,920.
US 2007/0221005 A1 discloses a forged composite gear and a method of making a forged composite powder metal gear. The forged composite gear includes a plurality of teeth extending from a core, a first section having a first powder metal material, a second section having a second powder material and a variable boundary profile. The variable boundary profile is formed between the first section and the second section, whereby the variable boundary profile exhibits greater tooth wear resistance on the teeth and greater impact resistance in the core. The powder metal material is, thus, arranged on a toothed section formed in the core.